In recent years, IC integration level has increased dramatically. This requires further minute precision hole processing, for example, accurately depth-processing 2–4 layers out of 5–6 layers of printed boards in the order of micro-meter, using a drill made of an ultra-hard material such as diamond having a diameter in the order of 0.1 mm or less. The revolution speed of the drill reaches hundreds of thousands per minute, causing abrasion to occur at the cutting edge. Therefore, the edge position must be checked regularly, e.g. every one hundred times of hole processing, in order to keep the relative position between the working table and the cutting edge constant.
A touch sensor is used as a checking apparatus for keeping the cutting edge stable with respect to a reference position provided as a predetermined height from the surface of the working table of the aforementioned machine tool. It is provided with a mechanical means for switching an electric contact from ON state to OFF state by a snap action of, e.g. toggle mechanism, when a free end of a lever-type actuator rotatably supported by a hinge is pressed against an edge tool, and its movement is transferred to a sliding pin. Alternatively, it may be provided with an optical means wherein the reference position of the edge is detected with a signal output at which the edge directly blocks the light beam, to correct the fluctuation due to the abrasion at the edge.
However, a contact load for activating a touch sensor having a mechanical means requires e.g. 100 to 300 gf as an energizing force in order to counteract resistance by anti-action accompanied with a snap action of a contact by moving the actuator lever. Accordingly, without such a contact pressure, a setting of a reference position with accuracy and stable reproducibility is not obtained. On one band, processing a diameter of 0.1 mm or less in the order of 0.01 mm using a diamond drill requires a touch sensor with a very light contact pressure, because activating a touch sensor with a contact load as described above may cause the drill itself to bend at a buckle, resulting in unstable position setting of the drill tip, thereby damaging reproducibility.
On the other hand, detecting directly the tip of a cutting tool by an optical method involves no contact and does not require a setting pressure. But it has a disadvantage that it is difficult to respond to variations in the cutting edge form, because the method has so many adjusting parts for making adjustments, which depend on the characteristics of the electronic circuit, such as adjustments of reproducibility, neutral zone, light amount, and temperature change, so that it is inconvenient for handling and vulnerable to dust.
Therefore, an object of the invention is to provide a touch sensor having a contact load of 0.5 gf or less as an operation force, reproducing sensitivity or precision in the range of ±0.5 μm, freedom of attaching to and detaching from the working table of the automated machine tool, and furthermore ease of handling the reference position setting.